What is Packet Radio

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PACKET RADIO

There are many different modes available to radio amateurs these days and
that is truely the beauty of our hobby. Packet Radio is one of those
modes and like the other modes in amateur it will appeal to a specific
group of people, especially those with an interest in computers and
networking.

For emergency uses, a wireless network of laptop computers and TNC's
with radios running on batteries can pass large volumes of traffic
and also provide other tactical uses such as access to a database
with information such as location of people and or emergency supplies.

WHAT IS PACKET RADIO?

Packet radio is communications fo the computer age. A computer in
a ham shack is as common as a 2 meter handheld transceiver was 20
years ago. Computer programs allowed computers to send and receive
CW and RTTY. Some farsighted hams, however, developed a new amateur
mode of communications that unleashes the power of the computer. The
mode is packet radio.

Being a child of the computer age, packet radio has the computer-age
features that you would expect.

* It is data communications; high speed and error-free packet
radio communications lends itself to the transfer of large
amounts of data.

* It is fast, much faster than the highest speed CW or RTTY.

* It is error free, no "hits" or "misses" caused by propagation
variations or electrical interference.

* It is spectrum efficient; several stations can share one
frequency at the same time.

* It is networking; packet stations can be linked together to
send messages over long distances.

* It is message storage; packet radio bulletin boards (PBBS)
provide storage of messages for later retrieval.

HOW DOES PACKET WORK?

Packet radio uses a terminal node controller (TNC) as the interface
between the computer and the transceiver. A TNC is an enhanced modem.
A modem is a device which converts the computers data into variable
audio tones and on the other end converts the variable audio tones
back to computer data. The TNC accepts information from your computer
or ASCII terminal and breaks data into small pieces called packets.
In addition to the information from your computer, each packet
contains addressing, error-checking and control information. The
addressing information includes the call sign of the station that
sent the packet, and the call sign of the station the packet is
being sent to. The address may also include call signs of stations
that are being used to relay the packet. The error-checking
information allows the receiving station to determine whether the
received packet contains any errors. If the received packet contains
errors, the receiving station asks for a repeat transmission until
the packet is received error free.

Breaking up the data into small parts allows several users to share
the frequency. Packets from one user are transmitted in the spaces
between packets from other users. The address section allows each
user's TNC to seperate packets intended for him from the packets
intended for other users. The addresses also allow packets to be
relayed through several stations before they reach their ultimate
destination. Having information in the packet that tells the
receiving station if the packet has been received correctly and
assures perfect copy.

PACKET RADIO REPEATING

Sometimes terrain or propagation prevents your signal from being
received by the other station. Packet radio gets around this
problem by using other packet radio stations to relay your
signal to ther intended station. All you need to know is which
on-the-air packet radio stations can relay signals between your
station and the station you want to contact. Once you know of
a station that can relay your signals you can use it for this
purpose.

Digital and voice repeaters repeat, but that is where the
similarity ends. Notice that digital repeaters differ from
typical voice repeaters in a number of ways. A digital repeater
(digipeater) usually receives and transmits on the same
frequency (whereas a voice repeater receives and transmits on
different frequencies). A digipeater does not receive and
transmit at the same time (as compared to a voice repeater,
which immediately transmits whatever it receives). Rather, a
digipeater receives a packet, stores it temporarily until
the frequency is clear, and then retransmits the packet. Also,
a digipeater only repeats packets that are specifically sent
to be repeated by that station (the address in the packet
contains the call sign of the digipeater). A voice repeater
repeats everything that it receives on its input frequency.

If one digipeater is insufficient to establish a connection,
you can specify as many as eight stations in your connect
request.

Don't use more than one or two digipeaters at any one time,
especially during prime time operating hours (evenings and
weekends). Each time you use a digipeater, you are competing
with other stations attempting to use the same digipeater.
Each station that you compete with has the potential of
generating a packet that may collide with your packet (which
causes your TNC to resend the packet). The more digipeaters
you use, the more stations you compete with, greatly increasing
the chance of a packet collision. As a result, it may be
difficult to get one packet through multiple digipeaters,
and your TNC will quickly reach its retry limit and disconnect
the link.

Any packet radio station can act as a digipeater. Most TNC's are
setup to digipeat automatically without any intervention by the
operator being used as a digipeater. You do not need permission,
only his cooperation, because he can disable his stations
digipeater function. In the spirit of Amateur Radio, most packet
operators leave the digipeat function on, disabling it only under
special circumstances.

Another form of a digipeater is a NODE. To reach a distant
station, first connect to the node. Then, instruct the node
to connect you to the distant station. The node acknowledges
packets sent from either station, then relays them to the
other station. This has a number of advantages over a simple
digipeater.

VHF/UHF VS. HF PACKET OPERATIONS

Today, most amateur radio packet activity occurs at VHF, on 2
meters, but activity on 430Mhz continues to grow as well.

The most common used data rate on VHF is 1200 baud with
frequency modulated AFSK tones of 1200 and 2200 Hz. This is
referred to as the "Bell 202" telephone modem standard.

Getting on the air is usually a simple matter of turning on
your radio amd tuning in your favorite packet radio frequency.
On 2 meters, common packet frequencies are 144.625, 144.675,
144.700. If there
is a voice repeater on that frequency in your area, ask around
at a club meeting or on the repeater. Someone is bound to know
where the packet activity is.

HF packet radio is very different from VHF/UHF packet. An SSB
transceiver is used to generate a 200Hz shift FSK signal, and
300 bauds is used rather than 1200 bauds. However, there is
some 1200 baud packet activity on the 10 meter band.

Tuning is much more critical than it is on VHF. Tune your
receiver very slowly, in as small an increment as possible until
your terminal begins displaying packets. Do not change frequency
until a whole packet is received. If you shift frequencies mid
packet, that packet will not be received properly and will not
be displayed on your terminal even if you were on the correct
frequency before or after the frequency shift.

Some TNCs and external modems have tuning indicators on them
that make tuning alot easier. Kits are also available to
allow you to add a tuning indicator to a TNC without one.

PACKET BULLETIN BOARD SYSTEMS (PBBS)

A Packet Bulletin Board System (PBBS) is a computer that allows
packet stations to store messages for other amateurs, upload and
download computer files, and even link one packet station through
a "gateway" to another band.

Some PBBS computers can automatically forward messages from one
computer to another, so you can store a message at one PBBS that
is ultimately meant for an amateur thousands of miles away. The
message will be forwarded from one PBBS to another until it
reaches its destination. A network is a system of packet stations
that can interconnect to transmit data over long distances.

To use a PBBS, you must locate one. If you are a member of R.A.A.G. and if the sysop
likes you and allows you to use his system, try the SV1SV PBBS.

THE DREADED BEACON

All TNC's have a beacon function. This function allows a station
to send an unconnected packet at regular intervals. These
unconnected packets usually contain a message to the effect that
the originating station is on the air and willing and able to
carry on a packet radio contact.

The purpose of the beacon function is to generate activity when
there is none. This purpose was legitimate when there was little
packet radio activity. Back in the early 1980's, it was a rare
occurence when a new packet station appeared on the air. Without
beacons, that new radio operator might believe that his packet
radio station was the only one active in the area. Similarly,
packet radio stations already on the air would not be aware
of the new stations existence. It would be very discouraging to
build a TNC (they were all kits in the early days), get on the
air and find no one to contact. The beacon function was a
solution to the problem. It let people know that a new packet
station was on the air.

Today, beacons are usually unnecessary. There is absoloutely
no need to resore to beaconing in order to make your stations
existence known. On HF, 2 meters and 430Mhz, there is plenty
of activity in most areas. After a few connections, your
existence on the air will be known.

Instead of sending beacons, leave a message announcing your
existence on the local PBBS. This is more effective than
sending beacons because your message will be read even when
your station is off the air.

Beacons only add congestion to already crowded packet radio
frequencies, so do the packet radio community a favor and
disable your TNC's beacon function using the following
command: BEACON EVERY 0

WHAT YOU NEED TO GET ON PACKET RADIO

All you need to set up a VHF/UHF packet radio station is a VHF/UHF
transceiver (with an antenna), a computer or ASCII terminal and
a TNC. The TNC connects between the computer and the radio. For
operation on 10 meters you will need a 10-meter SSB transceiver
in addition to the TNC and computer.
Your TNC manual should contain detailed instructions for wiring
the TNC, radio and computer together. So many hams are on packet
now that someone in the area will probably be able to
help you if you have problems, or ask around on the local voice
repeater.

PACKET IN YOUR AREA ??? NEED HELP ?????

There are many people on packet who would be willing to assist you
in your packet operations. Should you have any questions please contact
a local ham who is involved with packet radio, I am sure they will be
more than happy to assist you.